The present invention relates to rolling bearings and, more particularly, to an improvement of a rolling bearing used in automobiles, construction machinery, railroad vehicles, and other industrial machinery.
A variety of studies have heretofore been made to improve the life of rolling bearings. For example, it is most effective to decrease the content of oxygen in steel (hereinafter referred to as "oxygen content in steel") in order to reduce the presence of an oxide inclusion typically represented by alumina, which is one of the causes hampering the improvement of the rolling life of the bearings. Thus, to overcome this problem, many steelmakers are making efforts to improve various steelmaking technologies.
In the meantime, vacuum degassing and ladle refining furnace (LF) processes have been introduced in the field of bearing steels, and in the early 1980s any makers came to adopt steels made by continuous casing (hereinafter referred to as "CC steels") instead of steels made by ingot casting (hereinafter referred to as "IC steels") for their bearing steels.
In the continuous casting process for bearing steels, a large difference in the temperature of a steel exists between solidification start and solidification end, because the concentration of carbon in the steel is high. This has caused the problem that elements such as carbon, chromium, manganese, phosphorus, and sulfur are locally segregated and concentrated in the center portion of the steel to thereby shorten the rolling life of the bearings. Therefore, to improve the quality problems such as center segregation, flotation of inclusions, and unstable solidification structure, the development of sophisticated steelmaking technologies have been called for.
Recent developments such as an electromagnetic stirring process (EMS), a light reduction process with pinch rolls, and a continuous forging process have substantially eliminated the difference in rolling life between IC steel bearings and CC steel bearings.
For example, as described on page 288 of ASTM SPT987 (1988), from the result of a thrust life test carried out on samples cut out of a steel material in parallel with one another in the rolling direction including the center portion of the material, it has been verified that the rolling life of a rolling bearing made of a CC steel is rather longer than that of a rolling bearing made of an IC steel.
With respect to the relationship between flaking positions and rolling life, there has been no such report that the rolling life becomes shorter in the center porions of CC steels.
A report similar to the aforementioned has been made on page 116 of "Iron and Steel" (Vol. 73, No. 3, 1987).
Based on these reasons, many CC steels are used together with IC steels to make the inner and outer rings of rolling bearings in current production. The inner ring and the outer ring are made mainly of rods and tubes. The segregated center portion is hardly exposed to the functional surfaces of the bearing (particularly, to the groove surfaces) during the process of machining the steel material into the inner ring and the outer ring after lathing, hot forging, and warm forging.
That is, in the case of making the inner ring and the outer ring by hot forging and warm forging a rod, the center portion of the steel material is subjected to piercing, which removes the harmful center segregates.
Also, in the case of making the inner ring and the outer ring by lathing a tube, the segregated center portion on the inner-diameter surface of the tube is made harmless by lathing when the outer ring is made, because a large amount of such portion is lathed to make grooves. When the inner ring is made, the rolling life is not affected, because the segregated center portion coincides with the inner-diameter surface of the bearing.
From these points of view, CC steels have recently been adopted as the material of the inner and outer rings. That is, low cost, excellent cleanness, and excellent quality in streak flaw of the CC steels have prevailed over the center segregation problem thereof that has not yet been completely overcome.
On the other hand, with respect to the rolling elements, the most generally used material is a wire material. As shown in FIG. 6(b), the rolling elements are made by cold working (cold header working).
Specifically, as shown in FIG. 6(a), e.g., a steel material (billet) 20 is press-rolled to make a coil of steel strip 21.
Then, as shown in FIG. 6(b), the steel strip 21 is cut to a desired length by a cold header, and the cut pieces are formed to produce spherically formed products 23. Each formed product 23 has burrs 24. After removing the burrs by grinding, the formed product 23 is subjected to a heat treatment. The formed product 23 is ground and then lapped to obtain a rolling element 25 having a desired size.
In the thus formed rolling element 25 by cold working, the segregated center portion of the billet 20 is exposed at poles 30A and 30B of the rolling element 25 as shown in FIG. 7, i.e., at two functional surfaces. These poles 30A and 30B, combined with their exposure to the surface perpendicular to the rolling direction, become susceptible to flaking and cracking, which causes reduction in rolling life and deterioration in strength.
This has kept bearing makers from using CC steels having the problem of center segregation as the material of rolling elements. Therefore, IC steels are currently used as the material of rolling elements.
However, IC steels, requiring cutting of the nonuniform top and bottom portions of an ingot in the rolling process, are expensive compared with steels that are continuously cast.
In addition, IC steels are susceptible to streak flaws by chips of hot metal passage bricks and deoxidation products accidentally entrapped in the initially solidified layer during ingot making.
On the other hand, to improve the center segregation in CC steels, a continuous forging method disclosed in, e.g., Japanese Unexamined Patent Publications Nos. Hei. 3-254339, Hei. 3-254340, and Hei. 3-254341, and a light reduction process by pinch rolls disclosed in Japanese Unexamined Patent Publication No. Sho. 49-121738 are available. There is also a technique that can improve macrosegregation by causing the liquid steel having the concentrations of carbon, chromium, and the like to move up while deforming the CC strand with pressure applied thereto at the final stage of solidification during continuous casting.
However, such technique for improving the macrosegregation has a problem to be overcome. The problem is such that the relationship between the center segregation rate of carbon (C/C.sub.0) of a steel material (where C is the concentration of carbon in the center portion (wt. %) and C.sub.0 is the average concentration of carbon of the steel material (wt. %)) and the rolling life of the rolling element has not yet been fully analyzed.